In recent years, rechargeable lithium ion secondary batteries have become widely used as the power sources for driving vehicles including hybrid vehicles and electric vehicles, and portable electronic equipment such as notebook computers.
Lithium ion secondary batteries comprise, for example, a positive electrode plate having a positive electrode active material layer containing a positive electrode active material such as a lithium oxide that is capable of lithium insertion and removal, a binder and a conductive assistant formed on a positive electrode current collector, a negative electrode plate having a negative electrode active material layer containing a negative electrode active material such as a carbon material that is capable of lithium occlusion and release and a binder formed on a negative electrode current collector, and a non-aqueous electrolyte.
The positive electrode plate is obtained, for example, by coating the positive electrode current collector with a positive electrode paste containing the positive electrode active material, the binder, the conductive assistant and a solvent.
For example, Patent Document 1 discloses a production method for a positive electrode plate having a metal foil, and a positive electrode active material layer formed on this metal foil and containing positive electrode active material particles, a carbon-based conductive assistant and a binder, the production method comprising a positive electrode paste preparation step of preparing a positive electrode paste, and a coating step for coating the metal foil with this positive electrode paste, wherein during the positive electrode paste preparation step, the positive electrode paste is prepared with a solid content concentration selected from a range of 50 to 65 wt %, so that a set (x, y) of the value x for the mixed powder density of the mixed powder and the value y for the viscosity of the positive electrode paste falls within a prescribed region R on a semilogarithmic graph in which the horizontal axis represents the mixed powder density and the logarithmic scale vertical axis represents the viscosity of the positive electrode paste.
A higher solid content concentration for the electrode paste such as the positive electrode paste enables reductions in the drying time and the solvent volume when producing the electrode plate such as the positive electrode plate, and is consequently preferred. However, in those cases where the solid content concentration of the electrode paste is increased to a value that is higher than the level assumed in the prior art, such as a concentration exceeding 65% by mass, the viscosity of the electrode paste may sometimes become too high, making application of the paste with a coating device such as a die coater problematic. Accordingly, in the preparation of electrode pastes, there is still room for improvement from the viewpoint of further increasing the solid content concentration.